WordPress Menu Customization

/*
 *  Adding class in menu li
 */
add_filter('nav_menu_css_class', 'add_class_li', 10, 4);

function add_class_li($classes, $item, $args, $depth) {
    $classes[] = 'dropdown';
    return $classes;
}

/*
 *  Adding class in sub menu ul
 */
function new_submenu_class($menu) {
    $menu = preg_replace('/ class="sub-menu"/','/ class="yourclass" /',$menu);
    return $menu;
}

add_filter('wp_nav_menu','new_submenu_class');

Removing UL from wp_nav_menu

/* 
 * Removing UL from wp_nav_menu
 */
$left = wp_nav_menu( array('theme_location' => 'product-categories-left', 'echo' => false, 'container' => false, 'items_wrap' => '%3$s','fallback_cb'=> 'fall_back_menu' ));
			echo preg_replace(array(
				'#^<ul[^>]*>#',
				'#</ul>$#'
			), '', $left);